It is important that a mobile cellular station can get information about the cells surrounding the cell corresponding to its current location, so that it can move to the next cell (MAHO, Mobile Assisted Handover). The information which must be determined in this context is i.a. the signal levels, signal strengths and the synchronisation information of the base stations of neighbour cells. For instance in the GSM system (Global System for Mobile communications) the synchronisation information is the frequency correction channel (FCCH) and the synchronisation channel (SCH) of the neighbour cell. The object is to acquire the information without causing interference in the user information during a call.
In continuous transmission systems (CDMA, Code Division Multiple Access; FDMA, Frequency Division Multiple Access) and in time division systems (TDMA, Time Division Multiple Access), when all time slots are used for the communication of information, a problem is the second receiver required for the determination. In a TDMA system, such as the GSM system, where one or more idle frames are reserved for the determination of neighbour cell information, the idle time is used for reception. There is a problem in that the idle time is barely sufficient for the reception. Due to this it is not always possible to obtain the FCCH and SCH information during the idle time. On the average, this information is obtained only after several unsuccessful reception during the idle frames, when the location in time of the FCCH and the SCH is unknown, or it is necessary to wait over several idle frames, even when the location of the FCCH and the SCH is known.
Prior art is illustrated with the aid of FIG. 1. There the frames 101, 103 and 104 are normal TDMA information frames of the GSM system. The frame 102 is an idle frame, during which neighbour cell information is received 105 in a well known way. The time slots 0, 1, 2, . . . 7 of the information frames are used in the following way: the time slot 0 transmits received information RX of the downlink transmission direction: the time slot 3 transmits the transmitted information TX of the uplink transmission direction, and neighbour cell signal level measurements are made during the time slots 5 and 6 at a monitoring moment M, which is not tied to the time slots. The other time slots are used i.a. for changing the synthesiser frequency. RX and TX are information signals, which can be speech or data information. The so called discontinuous transmission method (DTX) is not used during the period shown in the figure.
When the SCH time slot appears in the determination period 105 it is read during the idle frame 102. However, as mentioned above, this happens very seldom, and in this way the time slot is not usually found at the first attempt. Several systematically unsuccessful attempts are required in order to success, if the FCCH and SCH locations in time are unknown, or a delay over several idle frames is required, even when the FCCH and SCH locations are known.